Hot stamped high strength part excellent in post painting anticorrosion property and method of production of same

ABSTRACT

A hot stamped high strength part in which the propagation of cracks which form at the plating layer at the time of hot stamping when hot stamping aluminum plated steel sheet is suppressed and the post painting anticorrosion property is excellent even without adding special ingredient elements which suppress formation of cracks in an aluminum plating layer is provided. 
     A hot stamped high strength part which is excellent in post painting anticorrosion property, which hot stamped high strength part has an alloy plating layer which includes an Al—Fe intermetallic compound phase on the surface of the steel sheet, wherein the alloy plating layer is comprised from phases of a plurality of intermetallic compounds, a mean linear intercept length of crystal grains of a phase containing Al: 40 to 65 mass % among the phases of the plurality of intermetallic compounds is 3 to 20 μm, an average value of thickness of the Al—Fe alloy plating layer is 10 to 50 μm, and a ratio of the average value of thickness to the standard deviation of thickness of the Al—Fe alloy plating layer satisfies the following relationship: 
       0&lt;standard deviation of thickness/average value of thickness≦0.15.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 14/008,854, filed Sep. 30, 2013, which is a national stageapplication of International Application No. PCT/JP2012/058655, filedMar. 30, 2012, which claims priority to Japanese Application No.2011-081995, filed Apr. 1, 2011, each of which is incorporated byreference in its entirety.

TECHNICAL FIELD

This application is a divisional application of U.S. application Ser.No. 14/008,854, filed Sep. 30, 2013, which is a national stageapplication of International Application No. PCT/JP2012/058655, filedMar. 30, 2012, which claims priority to Japanese Application No.2011-081995, filed Apr. 1, 2011, each of which is incorporated byreference in its entirety.

The present invention relates to an aluminum plated high strength partwhich is excellent in post painting anticorrosion property which isproduced by press forming at a high temperature, that is, by hotstamping, and is suitable for members in which strength is required suchas auto parts and other structural members, more specifically relates toa high strength part which is formed by hot stamping which is suppressedin propagation of cracks which form in the aluminum plating layer whenhot stamping aluminum plated high strength steel sheet and which isexcellent in post painting anticorrosion property, and a method ofproduction of the same.

BACKGROUND ART

In recent years, in applications of steel sheet for automobile use (forexample, automobile pillars, door impact beams, bumper beams, etc.) andthe like, steel sheet in which both high strength and high formabilityare achieved has been desired. As one means for dealing with this, thereis TRIP (transformation induced plasticity) steel which utilizes themartensite transformation of residual austenite. Using this TRIP steel,it is possible to produce high strength steel sheet which is excellentin formability and which has a 1000 MPa class or so strength, butsecuring formability with very high strength steel sheet of furtherhigher strength, for example, 1500 MPa or more, has been difficult.

In view of this situation, the forming method which has been focused onmost recently as a method for securing high strength and highformability has been hot stamping (also called hot pressing, hotstamping, die quenching, press quenching, etc.) This hot stamping heatsthe steel sheet to the 800° C. or higher austenite region, then forms itby a die when hot to thereby improve the formability of the highstrength steel sheet and, after forming it, cools it in the press die toquench it and thereby obtain a shaped part of the desired quality.

Hot stamping is promising as a method for forming very high strengthmembers, but usually includes a step of heating the steel sheet in theatmosphere. At this time, oxides (scale) form on the steel sheetsurface, so a later step of removing the scale becomes necessary. Inthis regard, in such a later step, there was the problem of the need formeasures from the viewpoint of the descaling ability and environmentalload etc.

As art to alleviate this problem, the art of using aluminum plated steelsheet as the steel sheet for hot stamped member use so as to suppressthe formation of scale at the time of heating has been proposed (forexample, see PLTs 1 and 2).

Aluminum plated steel sheet is effective for the efficient production ofa high strength shaped part by hot stamping. Aluminum plated steel sheetis usually pressed formed, then painted. The aluminum plating layerafter heating at the time of hot stamping changes to an intermetalliccompound up to the surface. This compound is extremely brittle. Ifsubjected to a severe forming operation by hot stamping, the aluminumplating layer easily cracks. Further, the phases of this intermetalliccompound have more electropositive potential than the matrix steelsheet, so there was the problem that the corrosion of the steel sheetmaterial is started from the cracks as starting points and the postpainting anticorrosion property falls.

To avoid the drop in the post painting anticorrosion property due to theformation of cracks in the aluminum plating layer, adding Mn to thisintermetallic compound is extremely effective, so an aluminum platedsteel sheet which is improved in post painting anticorrosion property byaddition of 0.1% or more of Mn in the aluminum plating layer has beenproposed (for example, see PLT 3).

The art which is described in PLT 3 adds specific ingredient elements inthe aluminum plating layer to prevent cracks from forming in thealuminum plating layer, but is not art which prevents cracks fromforming in the aluminum plating layer without addition of specificingredient elements into the aluminum plating layer.

Further, aluminum plated steel sheet has been proposed where, if addingelements to the matrix steel of the aluminum plated steel sheet to giveTi+0.1Mn+0.1Si+0.1Cr>0.25, these elements promote diffusion betweenAl—Fe so that even if cracks are formed in the aluminum plating layer,an Fe—Al reaction proceeds from around them and therefore the steelsheet material is prevented from being exposed and the corrosionresistance is improved (for example, see PLT 4).

However, the art which is described in PLT 4 does not try to preventcracks from forming at the aluminum plating layer.

CITATIONS LIST Patent Literature

PLT 1: Japanese Patent Publication No. 2003-181549A

PLT 2: Japanese Patent Publication No. 2003-49256A

PLT 3: Japanese Patent Publication No. 2003-34855A

PLT 4: Japanese Patent Publication No. 2003-34846A

SUMMARY OF INVENTION Technical Problem

The present invention was made in consideration of this situation andhas as its object the provision of a hot stamped high strength part inwhich the propagation of cracks which form at the aluminum plating layerwhen hot stamping aluminum plated steel sheet is suppressed and the postpainting anticorrosion property is excellent even without adding specialingredient elements which suppress formation of cracks in an aluminumplating layer. Further, it has as its object the formation of alubricating film at the aluminum plating layer surface to improve theformability at the time of hot stamping aluminum plated steel sheet andsuppress the formation of cracks in the aluminum plating layer.Furthermore, it has as its object the provision of a method ofproduction of a hot stamped high strength part.

Solution to Problem

The inventors engaged in intensive research to solve the above problemsand completed the present invention. In general, an aluminum platedsteel sheet for hot stamped member use is formed with an aluminumplating layer at one or both surfaces of the steel sheet by hot dippingetc. The aluminum plating layer may contain, by mass %, Si: 2 to 7% inaccordance with need and is comprised of a balance of Al and unavoidableimpurities.

When an aluminum plating layer of aluminum plated steel sheet before hotstamping contains Si, it is comprised of an Al—Si layer and Fe—Al—Silayer from the surface layer. To hot stamp an aluminum plated steelsheet, first, the aluminum plated steel sheet is heated to a hightemperature to make the steel sheet an austenite phase. Further, thealuminum plated steel sheet which is converted to austenite is pressformed hot, then the shaped aluminum plated steel sheet is cooled. Thealuminum plated steel sheet can be made a high temperature to make itsoften once and facilitate the subsequent press forming. Further, thesteel sheet may be heated and cooled so that it is quenched and anapproximately 1500 MPa or higher mechanical strength is realized.

In the heating step of this aluminum plated steel sheet for hot stampedmember use, inside the aluminum plating layer (when including Si), theAl—Si and the Fe from the steel sheet mutually diffuse thereby changingas a whole to an Al—Fe compound (intermetallic compound). At this time,in the Al—Fe compound, a phase which contains Si also is partiallyformed. This compound (intermetallic compound) is extremely brittle. Ifshaping it under severe conditions in hot stamping, cracks will form inthe aluminum plating layer. Further, these phases have a potential moreelectropositive than the matrix steel sheet, so corrosion of the steelsheet material will begin from the cracks as starting points and theshaped part will be reduced in post painting anticorrosion property.Therefore, suppression of the cracks which form in the aluminum platinglayer after hot stamping improves the post painting anticorrosionproperty of the part which is formed by hot stamping.

In hot stamping, it is not possible to avoid the formation of cracks inthe aluminum plating layer, but the inventors took note of the fact thatif it were possible to arrest the propagation of cracks of the aluminumplating layer which formed in hot stamping inside of the aluminumplating layer, the cracks would not reach the matrix steel sheet. Theydiscovered that this would enable prevention of corrosion of the steelsheet material and prevention of a detrimental effect on the postpainting anticorrosion property of the hot stamped part. The inventorsengaged in intensive research on arresting the propagation of cracks ofan aluminum plating layer for cracks which formed in the aluminumplating layer. As a result, they discovered that if controlling the meanlinear intercept length of crystal grains of an intermetallic compoundphase which contains Al in 40 to 65% among the crystal grains of theplurality of intermetallic compound phases based on Al—Fe which areformed at the surface of the steel sheet (below, sometimes simplyreferred to as the “mean linear intercept length”) to 3 to 20 μm, it ispossible to arrest the propagation of cracks which form in the aluminumplating layer. Further, they discovered that by further forming alubrication film which contains ZnO at the aluminum plating layersurface, it is possible to secure a lubricating property at the time ofhot stamping and possible to prevent surface defects and formation ofcracks. Furthermore, they discovered a steel sheet composition which issuitable for hot stamping.

Furthermore, the inventors discovered that the thickness of the Al—Fealloy plating layer has an effect on the state of spattering at the timeof spot welding and discovered that to obtain stable spot weldability,it is important to reduce the deviation of the plating thickness(standard deviation), make the average value of thickness of the Al—Fealloy plating layer 10 to 50 μm, and make the ratio of the average valueof thickness to the standard deviation of thickness (standard deviationof thickness/average value of thickness) 0.15 or less.

The present invention was completed based on these discoveries and hasas its gist the following:

(1) A hot stamped high strength part which is excellent in post paintinganticorrosion property, comprising an alloy plating layer comprising anAl—Fe intermetallic compound phase on the surface of the steel sheet,

the alloy plating layer is comprised from phases of a plurality ofintermetallic compounds,a mean linear intercept length of crystal grains of a phase containingAl: 40 to 65 mass % among the phases of the plurality of intermetalliccompounds is 3 to 20 μm, an average value of thickness of the Al—Fealloy plating layer is 10 to 50 μm, anda ratio of the average value of thickness to the standard deviation ofthickness of the Al—Fe alloy plating layer satisfies the followingrelationship:

0<standard deviation of thickness/average value of thickness≦0.15.

(2) The hot stamped high strength part which is excellent in postpainting anticorrosion property as set forth in the above (1)characterized in that the ratio of the average value of thickness to thestandard deviation of thickness is 0.1 or less.

(3) The hot stamped high strength part which is excellent in postpainting anticorrosion property as set forth in the above (1) or (2)characterized in that the Al—Fe alloy plating layer contains, by mass %,Si: 2 to 7%

(4) The hot stamped high strength part which is excellent in postpainting anticorrosion property as set forth in the above (1) or (2)characterized in that a surface film layer which contains ZnO isprovided on the surface of the Al—Fe alloy plating layer.

(5) The hot stamped high strength part which is excellent in postpainting anticorrosion property as set forth in the above (4)characterized in that a content of ZnO of the surface film layer is,converted to mass of Zn, 0.3 to 7 g/m² per side.

(6) The hot stamped high strength part which is excellent in postpainting anticorrosion property as set forth in the above (1) or (2)characterized in that the steel sheet is comprised of steel sheet ofchemical ingredients which comprise as ingredients, by mass %,

-   C: 0.1 to 0.5%,-   Si: 0.01 to 0.7%,-   Mn: 0.2 to 2.5%,-   Al: 0.01 to 0.5%,-   P: 0.001 to 0.1%,-   S: 0.001 to 0.1%,-   N: 0.0010% to 0.05%, and-   a balance of Fe and unavoidable impurities.

(7) The hot stamped high strength part which is excellent in postpainting anticorrosion property as set forth in the above (6)characterized in that the steel sheet further comprises, by mass %, oneor more elements selected from

-   Cr: over 0.4 to 3%,-   Mo: 0.005 to 0.5%,-   B: 0.0001 to 0.01%,-   W: 0.01 to 3%,-   V: 0.01 to 2%,-   Ti: 0.005 to 0.5%,-   Nb: 0.01 to 1%-   Ni: 0.01 to 5%,-   Cu: 0.1 to 3%,-   Sn: 0.005% to 0.1%, and-   Sb: 0.005% to 0.1%.

(8) A method of production of an aluminum plated steel sheet for a hotstamped high strength part, comprising steps of:

providing an aluminum plated steel sheet obtained characterized by

hot rolling a steel which comprises chemical ingredients which comprise,by mass %,

-   C: 0.1 to 0.5%,-   Si: 0.01 to 0.7%,-   Mn: 0.2 to 2.5%,-   Al: 0.01 to 0.5%,-   P: 0.001 to 0.1%,-   S: 0.001 to 0.1%,-   N: 0.0010% to 0.05%, and-   a balance of Fe and unavoidable impurities,

cold rolling said hot rolled steel to obtain a cold rolled steel sheet,

heating said cold rolled steel sheet on a hot dipping line to anannealing temperature of 670 to 760° C.,

holding said heated steel sheet in a reducing furnace for 60 sec orless, and

aluminum plating said steel sheet; and

temper rolling said aluminum plated steel sheet to give a rolling rateof 0.5 to 2%;raising the temperature of said temper rolled aluminum plated steelsheet by a temperature elevation rate of 3 to 200° C./sec; hot stampingthe aluminum plated steel sheet under conditions of a Larson-Millerparameter (LMP) expressed by the following formula:

LMP=T(20+log t)

(wherein, T: heating temperature of aluminum plated steel sheet(absolute temperature K), t: holding time in heating furnace afterreaching target temperature (hrs)) of 20000 to 23000; andquenching said aluminum plated steel sheet after hot stamping at a 20 to500° C./sec cooling rate in the die.

(9) The method of production of an aluminum plated steel sheet for a hotstamped high strength part as set forth in the above (8) characterizedin that the steel furthermore comprises, by mass %, one or more of theelements selected from

-   Cr: over 0.4 to 3%,-   Mo: 0.005 to 0.5%,-   B: 0.0001 to 0.01%,-   W: 0.01 to 3%,-   V: 0.01 to 2%,-   Ti: 0.005 to 0.5%,-   Nb: 0.01 to 1%-   Ni: 0.01 to 5%,-   Cu: 0.1 to 3%,-   Sn: 0.005% to 0.1%, and-   Sb: 0.005% to 0.1%.

(10) The method of production of an aluminum plated steel sheet for ahot stamped high strength part as set forth in the above (8) or (9)characterized in that in the temperature elevation rate in the hotstamping step is 4 to 200° C./sec.

(11) The method of production of an aluminum plated steel sheet for ahot stamped high strength part as set forth in any one of above (8) to(10) characterized in that in the step of producing the aluminum platedsteel sheet, a plating bath for aluminum plating comprises Si in anamount of 7 to 15%, and either a bath temperature or a sheet temperatureupon entering the bath is 650° C. or less.

Advantageous Effects of Invention

According to the present invention, it is possible to arrest crackswhich had formed in the plating layer (alloy layer) of aluminum platedsteel sheet at the time of hot stamping without allowing propagation atthe crystal grain boundaries of the plating layer. For this reason,cracks do not reach the surface of the hot stamped high strength partand the hot stamped high strength part can be improved in post paintinganticorrosion property. Further, in the present invention, the surfaceof the plating layer of the aluminum plated steel sheet is furtherformed with a lubricating surface film layer which contains ZnO and thenthe sheet is hot stamped to obtain the shaped part. Due to this, it ispossible to improve the workability at the time of hot stamping andpossible to suppress the formation of cracks, so the productivity can beraised. Furthermore, by reducing the deviation of the plating thickness,the spot weldability can be stabilized. Further, by using a steel sheethaving the steel ingredients of the present invention, it is possible toobtain a hot stamped high strength part which has a 1000 MPa or moretensile strength.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a polarization micrograph of the structure of an aluminumplating layer at the cross- section of a hot stamped part.

FIG. 2 is an Al—Fe—Si ternary phase diagram (650° C. isotherm).

FIGS. 3( a) to (d) are polarization micrographs of the structure of analuminum plating layer. (a) shows the case of a plating thickness of 40g/m per side and a temperature elevation rate at hot stamping of 5° C.(b) shows the case of a plating thickness of 40 g/m per side and atemperature elevation rate at hot stamping of 20° C. (c) shows the caseof a plating thickness of 80 g/m per side and a temperature elevationrate at hot stamping of 5° C. (d) shows the case of a plating thicknessof 80 g/m per side and a temperature elevation rate at hot stamping of20° C. Further, (a) is a view which shows the method of finding the meanlinear intercept length of crystal grains by the line segment method. Itis a view which shows the mean linear intercept length found by drawinga line parallel to the plating layer surface, counting the number ofgrain boundaries which are passed by through this line, and dividing themeasured length by the number of grain boundaries. In (a), the meanlinear intercept length was 12.3 μm.

FIG. 4 is a view which shows the effects of the aluminum platingconditions and heating conditions at the time of hot stamping on themean linear intercept length of an intermetallic compound phase whichcontains Al: 40 to 65%. The abscissa shows the Larson-Miller parameter(LMP) of the heating conditions at the time of hot stamping.

FIG. 5 is a polarization micrograph of the structure of the aluminumplating layer of FIG. 3 wherein the grain boundaries of the crystalgrains are traced to clearly show them.

FIG. 6 is a view which shows the relationship between the amount ofdeposition of Zn on the aluminum plated steel sheet surface and thedynamic coefficient of friction.

DESCRIPTION OF EMBODIMENTS

The hot stamped part of the present invention is made a high strengthpart by plating the surface of steel sheet with Al, heat treating theobtained aluminum plated steel sheet to make the aluminum plating layerform an alloy down to the surface, and then hot stamping it.

The method of aluminum plating in the aluminum plated steel sheet forhot stamped member use which is used in the present invention is notparticularly limited. For example, the hot dipping method, first andforemost, and also the electroplating method, vacuum deposition method,cladding method, etc. may be used, but currently the plating methodwhich is most prevalent industrially is the hot dipping method. Thismethod is desirable. Usually, in aluminum plating of steel sheet, analuminum plating bath which contains 7 to 15 mass % of Si can be used,but Si need not necessarily be contained. Si acts to suppress the growthof the alloy layer of the aluminum plating at the time of plating. Iflimited to hot stamping applications, there is little need to suppressgrowth of the alloy layer, but in the hot dipping method, a single bathis used to produce products for various applications, so in applicationswhere workability of the aluminum plating is demanded, alloy layergrowth has to be suppressed, so Si is usually included. In the presentinvention, the amount of Si which is contained in the aluminum platinglayer before the aluminum plating layer becomes alloyed, as explainedlater, is the factor which governs the mean linear intercept length ofthe Al—Fe alloy. In the present invention, the aluminum plating bathpreferably includes Si: 7 to 15%. By heating the aluminum plating layerto make it become alloyed at the time of hot stamping, Fe diffuses fromthe steel sheet material into the plating layer and the concentration ofSi in the Al—Fe falls compared with the inside of the aluminum platinglayer before hot stamping. If the aluminum plating bath contains 7 to15% of Si, the Al—Fe alloy layer after hot stamping contains Si in anamount of 2 to 7%.

The steel sheet in the hot stamped high strength part of the presentinvention has an Al—Fe alloy layer formed by alloying of the aluminumplating at the surface due to annealing at the time of hot stamping.This Al—Fe alloy layer has an average value of thickness of 10 to 50 μm.If the thickness of this Al—Fe alloy layer is 10 μm or more, after theheating step, sufficient post painting anticorrosion property cannot besecured by the aluminum plated steel sheet for rapidly heated hotstamped member use. The greater the thickness, the better in terms ofthe corrosion resistance, but the greater the thickness of the Fe—Alalloy layer, the easier it is for the surface layer to drop off at thetime of hot stamping, so the upper limit of the average value ofthickness is made 50 μm or less.

Further, deviation in the thickness of the Al—Fe alloy layer of a hotstamped high strength part affects the stability of spot weldability.According to studies of the inventors, the thickness of the Al—Fe alloylayer affects the spattering current value. The smaller the deviation inthickness, the lower the spattering current as a general trend. For thisreason, if the deviation in thickness of the Al—Fe alloy layer is large,the spattering current value easily varies and as a result the range ofsuitable welding current becomes smaller. Therefore, it is necessary tosuitably control the deviation in thickness of the Al—Fe alloy layer. Itwas learned that it was necessary to make the ratio of the average valueof thickness to the standard deviation of thickness (standard deviationof thickness/average value of thickness) of the Al—Fe alloy platinglayer 0.15 or less. More preferably, the ratio is 0.1 or less. By doingthis, stable spot weldability is obtained.

The thickness of the Al—Fe alloy plating layer of a hot stamped highstrength part was measured and the standard deviation of thickness wascalculated by the following procedure. First, steel was hot rolled, thencold rolled and was coated with Al by a hot dipping line. The entirewidth of the steel sheet was heated and quenched. After that, atpositions 50 mm from the two edges in the width direction, the center ofwidth, and intermediate positions of the positions 50 mm from the twoedges and the center, a total of five locations, 20×30 mm test pieceswere sampled. The test pieces were cut, the cross-sections wereexamined, and the thicknesses at the front and back were measured. Atthe cross-sections of the test pieces, any 10 points were measured forthickness. The average value of thickness and the standard deviation ofthickness were calculated. In the measurement of the thickness at thistime, each cross-section was polished, then was etched by 2 to 3% Nitalto clarify the interface between the Al—Fe alloy layer and the steelsheet and measure the thickness of the alloy plating layer.

When the aluminum plating layer of the aluminum plated steel sheetbefore hot stamping contains Si, the layer is comprised of the twolayers of the Al—Si layer and Fe—Al—Si layer in order from the surfacelayer. If this Al—Si layer is heated in the hot stamping step to 900° C.or so, Fe diffuses from the steel sheet, the plating layer as a wholechanges to a layer of Al—Fe compound, and a layer which partiallycontains Si in the Al—Fe compound is also formed.

It is known that when heating aluminum plated steel sheet to alloy thealuminum plating layer before hot stamping, the Fe—Al alloy layergenerally usually has a five-layer structure. Among these five layers,in order from the coated steel sheet surface layer, the first layer andthe third layer mainly comprise Fe₂Al₅ and FeAl₂. In those layers, theconcentrations of Al are approximately 50 mass %. The concentration ofAl in the second layer is approximately 30 mass %. The fourth layer andthe fifth layer can be judged to be layers corresponding to FeAl andaFe. The concentrations of Al in the fourth layer and the fifth layerare respectively 15 to 30 mass % and 1 to 15 mass %, that is, broadranges in the compositions. The balance was Fe and Si in each layer.These alloy layers had corrosion resistances substantially dependent onthe Al content. The higher the Al content, the better the corrosionresistance. Therefore, the first layer and the third layer are the bestin corrosion resistance. Note that, below the fifth layer is the steelsheet martensite. This is a hardened structure mainly comprised ofmartensite. Further, the second layer is a layer which contains Si whichcannot be explained from the Fe—Al binary phase diagram. The detailedcomposition is not clear. The inventors guess that this is a phase whereFe₂Al₅ and Fe—Al—Si compounds are finely mixed.

When rapidly heating and hot stamping such aluminum plated steel sheet,the structure of the obtained Al—Fe alloy layer, while depending on theheating conditions at the time of hot stamping, does not exhibit such aclear five-layer structure. This believed because since rapid heating isinvolved, the amount diffusion of Fe into the plating layer is small.

The Al—Fe alloy layer is formed by the diffusion of the Fe in the steelsheet material into the aluminum plating, so has a distribution ofconcentration where the concentration of Fe is high and theconcentration of Al is low at the steel sheet side of the aluminumplating layer and, further, the concentration of Fe falls and theconcentration of Al rises toward the surface side of the plating layer.

If examining the aluminum plating layer of a hot stamped part, since theAl—Fe alloy phase is hard and brittle, cracks form in the plating layerof the hot stamped part. FIG. 1 is a polarization micrograph of thestructure of an aluminum plating layer at the cross-section of a hotstamped part. As shown in FIG. 1, it is learned that large cracks runthrough the crystal grains and reach the matrix, so small cracks arearrested at the crystal grain boundaries (shown by arrow).

Therefore, the inventors took note of the phenomenon of cracks beingarrested at the crystal grains boundaries and studied in depth thearrest of propagation of cracks which form at the aluminum platinglayer. As a result, they discovered that by controlling, among thecrystal grains of the plurality of intermetallic compound layers mainlycomprised of Al—Fe which are formed at the surface of the steel, theaverage intercept layer of the crystal grains of an intermetalliccompound layer which contains Al: 40 to 65% to 3 to 20 μm in range, itis possible to arrest the propagation of cracks which form at thealuminum plating layer. As explained below, the “mean linear interceptlength” referred to here means the length measured in a directionparallel to the surface of the steel sheet. Here, the alloyed aluminumplating naturally is mainly comprised of Al and Fe, but the aluminumplating also contains Si, so it is mainly comprised of Al—Fe andcontains a small amount of Al—Fe—Si.

The inventors studied the dominating factors which affect the meanlinear intercept length of a phase which contains Al: 40 to 65%,whereupon they found that the mean linear intercept length of a phasewhich contains Al: 40 to 65% is greatly affected by the platingthickness, the heat history (temperature elevation rate and holdingtime), the aluminum plating conditions (amount of Si, bath temperature,and sheet temperature when dipped) and other manufacturing conditions ofhot stamped high strength parts. Specifically, the effect of the type ofalloy layer after aluminum plating is particularly large. The heathistory can be controlled by using the Larson-Miller parameter (LMP)which is explained below.

To reduce the mean linear intercept length of a phase which contains Al:40 to 65% after alloying to a finer 3 to 20 μm, it is preferable to formβ-AlFeSi as the initial alloy layer at the time of aluminum plating.β-AlFeSi is a compound which has a monoclinic crystal structure and isalso said to have a composition of Al₅FeSi. Furthermore, to formβ-AlFeSi as the alloy layer after aluminum plating, it is effective tomake the amount of Si in the bath 7 to 15% and the bath temperature 650°C. or less or to make the bath temperature 650 to 680° C. and the sheettemperature upon entry 650° C. or less. This is because at the Siconcentration and temperature of this region, β-AlFeSi becomes a stablephase.

The reason why the mean linear intercept length of a phase whichcontains

Al: 40 to 65% becomes small when forming β-AlFeSi as an alloy layerafter aluminum plating can be deduced from the Al—Fe—Si ternary phasediagram which is shown in FIG. 2. A phase which contains Al: 40 to 65%is believed to be a phase which mainly comprises Fe₂Al₅. The phase of acompound in an alloy layer which is formed by aluminum plating is aphase which balances with a liquid phase of Al—Si and can take threeforms of an α-phase, β-phase, and FeAl₃-phase. For example, when anFeAl₃ phase is formed, if Fe diffuses in this compound, it is believedthat the FeAl₃ phase transforms to an Fe₂Al₅ phase. As opposed to this,for the β-phase to be transformed in phase to Fe₂Al₅, it is necessary togo through numerous transformations such as β-phase->α-phase->FeAl₃phase->Fe₂Al₅ phase. By going through the transformations, crystalgrains are formed again, so the greater the transformations which aregone through, the smaller the mean linear intercept length tends tobecome. That is, the mean linear intercept length becomes smaller withthe α-phase than the FeAl₃ phase and with the β-phase than the α-phase.

The method of measurement of a mean linear intercept length in an alloyplating layer is to polish any cross-section of a hot stamped part, thenetch it by 2 to 3 vol % of Nital and examine the result by a microscope.For the examination, a polarization microscope is used. The polarizationangle is adjusted so that the contrast of the crystal grains becomes theclearest. At this time, the layer of a compound whose contrast appearslight at the surface layer side consecutively from the layer of acompound whose contrast appears dark is a phase of Al: 40 to 65%. Thisphase is a phase which has the property of arresting the crackpropagation and is a phase which affects the post painting anticorrosionproperty and the plating workability. As shown in FIGS. 3( a) to (b), inparticular when the plating thickness is thin (40 g/m² per side), due tothe effect of the dark contrast phase, the mean linear intercept lengthof Al: 40 to 65% phase is difficult to measure. Therefore, in thisDescription, the mean linear intercept length of the crystal grains inthe alloy plating layer is defined as the mean linear intercept lengthwhich is measured in the direction parallel to the steel sheet surface.The mean linear intercept length is found by the line segment method. Asshown in FIG. 3( a), the mean linear intercept length is found bydrawing a line parallel to the steel sheet surface in the plating layer,counting the number of grain boundaries which this line passes through,and dividing the measured length by the number of grain boundaries. Itis possible to calculate the grain size from this mean linear interceptlength, but calculation of the grain size requires that the shape of thegrains be known. In steel sheet, crystal grains can be assumed to bespherical, but the intermetallic compounds which are formed at thesurface like in the present invention are unknown in crystal grainshape, so the mean linear intercept length was used.

Note that, in actual measurement, in the polarization micrographs ofFIGS. 3( a) to (d), the grain boundaries are unclear, so as shown inFIGS. 5( a) and (b), the crystal grain boundaries were traced in thepolarization micrographs of FIGS. 3( a) and (c) to clarify the crystalgrain boundaries.

The reason for limiting the mean linear intercept length of a phasewhich contains Al: 40 to 65% after the aluminum plating layer is alloyedto 3 to 20 μm will be explained. A small grain size is preferable as acrack propagation arrest property of a phase which contains Al: 40 to65%, but the steel sheet for hot stamping member use has to be heatedonce to the austenite region. For this reason, this steel sheet isgenerally heated to 850° C. or more, so the aluminum plating layer whichis alloyed in this heating step ends up with crystal grains growing to 3μm or more. Therefore, usually making the crystal grain size less than 3μm is extremely difficult. If the mean linear intercept length exceeds20 μm and the grain size becomes larger, the aluminum plating layerfalls in workability and the phenomenon of powdering becomes greater.Furthermore, the crack propagation arrest property of a phase whichcontains Al: 40 to 65% no longer functions and cracks can no longer bearrested by the crystal grains.

Therefore, in the present invention, the mean linear intercept length ofa phase which contains Al: 40 to 65% was limited to 3 to 20 μm,preferably it is 5 to 17 μm.

Next, the effects of the aluminum plating conditions and heatingconditions at the time of hot stamping on the mean linear interceptlength will be explained.

FIG. 4 is a view which shows the effects of the aluminum platingconditions and the heating conditions at the time of hot stamping on themean linear intercept length. In FIG. 4, the abscissa shows theLarson-Miller parameter (LMP) of the heating conditions at the time ofhot stamping.

The Larson-Miller parameter (LMP) is expressed by

LMP=T(20+log t)

(wherein, T: absolute temperature (K), t: time (hrs)). Here, T is theheating temperature of the steel sheet, while “t” is the holding time inthe heating furnace after reaching the target temperature. LMP is anindicator which is used in general for treating the temperature and timein a unified manner in heat treatment and phenomena such as creep wherethe temperature and time have an effect. This parameter can also be usedfor the growth of crystal grains. In the present invention, LMPsummarizes the effects of temperature and time on the mean linearintercept length of crystal grains, so the heat treatment conditions atthe time of hot stamping can be described by just this parameter.

The symbols which are described in FIG. 4 will be explained. A and Bshow aluminum plating conditions. A means a 7% Si bath of a bathtemperature of 660° C., while B means a 11% Si bath of a bathtemperature of 640° C. These are typical conditions whereby an α-AlFeSiphase and a β-AlFeSi phase are produced at the time of aluminum plating.Further, “5° C./s” and “50° C./s” mean the temperature elevation ratesat the time of hot stamping. 5° C./s corresponds to usual furnaceheating, while 50° C./s corresponds to infrared heating, ohmic heating,and other rapid heating. Here, the “temperature elevation rate” meansthe average temperature elevation rate from the start of temperatureelevation to a temperature 10° C. lower than the target temperature. Ifcomparing the aluminum plating conditions A and B, the trend is thatforming an α-AlFeSi phase at the time of the conditions A, that is,aluminum plating, gives a mean linear intercept length greater than theconditions B. It was learned that it is necessary to limit the range ofheating conditions at the time of hot stamping to a narrower range(LMP=20000 to 23000). If the LMP is less than 20000, the diffusion ofthe Al—Si plating layer with the steel sheet is insufficient and anunalloyed Al—Si layer remains, so this is not preferred. Further, in theplating conditions A of FIG. 4, comparing the temperature elevationrates of 5° C./sec and 50° C./sec, it is shown that even with such anarrow range, if increasing the temperature elevation rate at the hotstamping, the structure becomes finer. The temperature elevation rate ispreferably 4 to 200° C./sec(s) in range. If the temperature elevationrate is slower than 4° C./sec, this means that the heating step takestime and means that the hot stamping falls in productivity. Further, iffaster than 200° C./sec, control of the temperature distribution in thesteel sheet becomes difficult. Both are not preferred. Establishingsuitable aluminum plating conditions and hot stamping conditions enablesthe mean linear intercept length to be made 3 to 20 μm.

As explained above, by making the mean linear intercept length of thecrystal grains of a phase containing Al: 40 to 65% in the layer of theintermetallic compounds mainly comprised of Al—Fe which is formed at thesurface of the steel 3 to 20 μm, it is possible to arrest thepropagation of cracks which form at the plating layer due to hotstamping inside the plating layer. Due to this, it is possible tosuppress corrosion of the steel sheet matrix due to cracks in theplating layer and possible to obtain high strength auto parts which areexcellent in post painting anticorrosion property and other hot stampedparts.

The hot stamped high strength parts of the present invention further mayhave a surface film which contains ZnO at the surface of the alloyplating layer mainly comprised of Al—Fe.

The hot stamped high strength part of the present invention has theextremely hard Al—Fe intermetallic compounds formed at the plating layerof the steel sheet surface at the time of hot stamping. For this reason,working defects are formed at the surface of the shaped part due tocontact with the die at the time of press forming in the hot stamping.There is the problem that these working defects because the cause ofcracks in the plating layer. The inventors discovered that by forming asurface film which has excellent lubricity at the surface of thealuminum plating layer, it is possible to suppress the working defectsof a shaped part and the occurrence of cracks in the plating layer anddiscovered that it is possible to improve the formability at the time ofhot stamping and the corrosion resistance of a shaped part.

The inventors engaged in intensive studies on a surface film which haslubricity which is suitable for hot stamping and as a result discoveredthat providing the surface of the aluminum plating layer with alubricating surface film layer which contains ZnO (zinc oxide), it ispossible to effectively prevent working defects of the shaped partsurface and cracks in the plating layer.

ZnO is included in the surface film layer at one side of the aluminumplated steel sheet in an amount, converted to mass of Zn, of 0.3 to 7g/m². More preferably, it included in 0.5 to 4 g/m². If the content ofZnO is, converted to mass of Zn, 0.1 g/m² or more, the effect ofimprovement of the lubricity and effect of prevention of segregation(effect of enabling uniform thickness of aluminum plating layer) etc.can be effectively exhibited. On the other hand, when the content of ZnOexceeds, converted to mass of Zn, 7 g/m², the total thickness of thealuminum plating layer and surface film layer becomes too thick and theweldability or painting adhesion falls.

FIG. 6 is a view which shows the relationship between the amount ofdeposition of Zn on the aluminum plated steel sheet surface and thecoefficient of dynamic friction. The content of ZnO in the surface filmlayer was changed to evaluate the lubricity at the time of hot stamping.This lubricity was evaluated by the following test. First, differenttest materials of the aluminum plated steel sheet which has an ZnO filmlayer (150×200 mm) were heated to 900° C., then were cooled down to 700°C. The test materials were subjected to loads from above through steelballs. Further, the steel balls were slid out over the test materials.At this time, the pullout load was measured by a load cell. The ratio ofthe pullout load/push-in load was made the coefficient of dynamicfriction. The results are shown in FIG. 6. If the coefficient of dynamicfriction is smaller than 0.65, it is evaluated as good. It is learnedthat in a region where the amount of deposition of Zn is generally 0.7g/m² or more, the coefficient of dynamic friction is effectively keptlow and the hot lubricity can be improved.

A surface film layer which contains ZnO can be formed, for example, byapplying a paint which contains ZnO and baking or drying it afterapplying for curing so as to enable formation over the aluminum platinglayer. As the method of applying a ZnO paint, for example, the method ofmixing a predetermined organic binder and a dispersion of ZnO powder andapplying it to the surface of the aluminum plating layer, a method ofpainting by powder painting, etc. may be mentioned. As the method ofbaking and drying after applying, for example, a hot air furnace,induction heating furnace, near infrared ray furnace, or other method ora method combining the same may be mentioned. At this time, depending onthe type of the binder which is used for applying, instead of baking anddrying after applying, for example, curing by ultraviolet rays orelectron beams etc. is possible. As the predetermined organic binder,for example, a polyurethane resin or polyester resin etc. may bementioned. However, the method of forming the ZnO surface film layer isnot limited to these examples and can be formed by various methods.

Such a surface film layer which contains ZnO can improve the lubricityof an aluminum plated steel sheet at the time of hot stamping, soworking defects of the plating layer and cracks in the plating layer atthe surface of the shaped part can be suppressed.

ZnO has a melting point of approximately 1975° C. or higher comparedwith the aluminum plating layer (the melting point of aluminum isapproximately 660° C.) etc. Therefore, even when working steel sheet atfor example 800° C. or more such as when working a coated steel sheet bythe hot stamping method etc., the surface film layer which contains thisZnO will not melt. Therefore, even if heating of the aluminum platedsteel sheet causes the aluminum plating layer to melt, the state wherethe ZnO surface film layer covers the aluminum plating layer to bemaintained, so it is possible to prevent the thickness of the meltedaluminum plating layer from becoming uneven. Note that, uneven thicknessof the aluminum plating layer of a hot stamped high strength part easilyoccurs, for example, in the case of heating by a furnace where the blankis oriented vertically with respect to the direction of gravity or thecase of heating by ohmic heating or induction heating. However, thissurface film layer can prevent uneven thickness of the aluminum platinglayer when such heating is performed and enables aluminum plating layerto be formed thicker.

In this way, an ZnO surface film layer exhibits the effects of improvingthe lubricity and making the thickness of the aluminum plating layeruniform etc. so can improve the formability at the time of press formingin hot stamping and the corrosion resistance after press forming.

Further, the aluminum plating layer can be made uniform in thickness, socan be rapidly heated by ohmic heating or induction heating enabling ahigher temperature elevation rate. This is effective for making the meanlinear intercept length of the crystal grains of an intermetalliccompound phase which contains Al: 40 to 6 5 mass % 3 to 20 μm.

Furthermore, this ZnO surface film layer never causes a drop in the spotweldability, paint adhesion, post painting anticorrosion property, andother performance. The post painting anticorrosion property is ratherfurther improved by imparting a surface film layer.

Next, the inventors studied the composition of ingredients for steelsheet for obtaining the aluminum plated steel sheet for rapidly heatedhot stamped member use provided with both excellent corrosion resistanceand excellent productivity. As a result, since the hot stamping wasperformed with the pressing and quenching simultaneously by the die,they obtained the ingredients for the steel sheet which are explainedbelow from the viewpoint of the aluminum plated steel sheet for hotstamped member use containing ingredients enabling easy quenching andthereby giving hot stamped parts which have a 1000 MPa or more highstrength after hot stamping.

Below, the reasons for limiting the ingredients of the steel sheet inthe present invention will be explained. Note that, the % of theingredients mean mass %.

C: 0.1 to 0.5%

The present invention provides a hot stamped part which has a 1000 MPaor more high strength after shaping. To obtain high strength, the steelhas to be rapidly cooled after hot stamping to transform it to astructure of mainly martensite. From the viewpoint of improvement of thehardenability, an amount of C of at least 0.1% is necessary. On theother hand, if the amount of C is too great, the toughness of the steelsheet remarkably falls, so the workability falls. For this reason, theamount of C is preferably 0.5% or less.

Si: 0.01 to 0.7%

Si promotes a reaction between the Al and Fe in the plating and has theeffect of raising the heat resistance of the aluminum plated steelsheet. However, Si forms a stable oxide film during therecrystallization annealing of the cold rolled steel sheet at the steelsheet surface, so is an element which obstructs the properties of thealuminum plating. From this viewpoint, the upper limit of the amount ofSi is made 0.7%. However, if making the amount of S less than 0.01%, thefatigue property deteriorates, so this is not preferable. Therefore, theamount of Si is 0.01 to 0.7%.

Mn: 0.2 to 2.5%

Mn is well known as an element which raises the hardenability of steelsheet. Further, it is also an element which is necessary for preventinghot embrittlement due to the unavoidably entering S. For this reason,0.2% or more has to be added. Further, Mn raises the heat resistance ofsteel sheet after aluminum plating. However, if over 2.5% of Mn isadded, the part which is hot stamped after quenching falls in impactproperties, so 2.5% is made the upper limit.

Al: 0.01 to 0.5%

Al is suitable as a deoxidizing element, so 0.01% or more may beincluded. However, if included in a large amount, coarse oxides areformed and the mechanical properties of the steel sheet are impaired, sothe upper limit of the amount of Al is made 0.5%.

P: 0.001 to 0.1%

P is an impurity element which is unavoidably included in steel sheet.However, P is a solution strengthening element. It can raise thestrength of the steel sheet relatively inexpensively, so the lower limitof the amount of P was made 0.001%. However, if recklessly increasingthe amount of addition, the toughness of the high strength material islowered and other detrimental effects appear, so the lower limit of theamount of P was made 0.1%.

S: 0.001 to 0.1%

S is an unavoidably included element. It forms inclusions of MnS in thesteel. If the MnS is large in amount, the MnS forms starting points offracture, obstructs ductility and toughness, and becomes a cause ofdeterioration of workability. Therefore, the amount of S is preferablyas low as possible. The upper limit of the amount of S was made 0.1% orless, but reducing the amount of S more than necessary is not preferablefrom the viewpoint of manufacturing costs, so the lower limit was made0.001%.

N: 0.0010% to 0.05%

N easily bonds with Ti or B, so has to be controlled so as not todecrease the effects targeted by these elements. An amount of N of 0.05%or less is allowable. Preferably, the amount of N is 0.01% or less. Onthe other hand, reduction more than necessary places a massive load onthe steelmaking step, so 0.0010% should be made the target for the lowerlimit of the amount of N.

Next, the ingredients which can be selectively contained in the steelwill be explained.

Cr: over 0.4% to 3%

Cr is also an element which generally raises the hardenability. It isused in the same way as Mn, but also has a separate effect when applyingan aluminum plating layer to steel sheet. If Cr is present, for example,when box annealing the steel after applying the aluminum plating layerso as to alloy the aluminum plating layer, the plating layer and thesteel sheet matrix easily alloy with each other. When box annealing thealuminum plated steel sheet, AlN is formed in the aluminum platinglayer. AlN suppresses the alloying of the aluminum plating layer andleads to peeling of the plating, but addition of Cr makes formation ofAlN difficult and makes alloying of the aluminum plating layer easier.To obtain these effects, the amount of Cr is over 0.4%. However, even ifadding Cr in an amount of over 3%, the effect becomes saturated.Further, the cost also rises. In addition, the aluminum plating propertyfalls. Therefore, the upper limit of the amount of Cr is 3%.

Mo: 0.005 to 0.5%

Mo, like Cr, has the effect of suppressing the formation of AlN, whichcauses peeling of the plating layer, formed at the interface of theplating layer and the steel sheet matrix when box annealing the aluminumplating layer. Further, it is a useful element from the viewpoint of thehardenability of the steel sheet. To obtain these effects, an amount ofMo of 0.005% is necessary. However, even if adding over 0.5%, the effectbecomes saturated, so the upper limit of the amount of Mo is 0.5%.

B: 0.0001 to 0.01%

B also is a useful element from the viewpoint of the hardenability ofsteel sheet and exhibits its effect at 0.0001% or more. However, even ifadding over 0.01%, the effect becomes saturated and, further, castingdefects and cracking of the steel sheet at the time of hot rolling occuretc. and the manufacturability otherwise drops, so the upper limit ofthe amount of B is 0.01%. Preferably, the amount of B is 0.0003 to0.005%.

W: 0.01 to 3%

W is a useful element from the viewpoint of the hardenability of steelsheet and exhibits its effect at 0.01% or more. However, even if over 3%is added, the effect becomes saturated and, further, the cost alsorises, so the upper limit of the amount of W is 3%.

V: 0.01 to 2%

V, like W, is a useful element from the viewpoint of the hardenabilityof steel sheet and exhibits its effect at 0.01% or more. However, evenif V us added in an amount over 3%, the effect becomes saturated and,further, the cost also rises, so the upper limit of the amount of V is2%.

Ti: 0.005 to 0.5%

Ti can be added from the viewpoint of fixing the N. By mass %, Ti has tobe added in an amount of approximately 3.4 times the amount of N, but N,even if decreased, is present in 10 ppm or so, so the lower limit of theamount of Ti was made 0.005%. Further, even if excessively adding Ti,the hardenability of the steel sheet is caused to fall or the strengthis also caused to fall, so the upper limit of the amount of Ti is 0.5%.

Nb: 0.01 to 1%

Nb, like Ti, can be added from the viewpoint of fixing the N. By mass %,Nb has to be added in an amount of approximately 6.6 times the amount ofN, but N, even if decreased, is present in 10 ppm or so, so the lowerlimit of the amount of Nb was made 0.01%. Further, even if excessivelyadding Nb, the hardenability of the steel sheet is caused to fall or thestrength is also caused to fall, so the upper limit of the amount of Nbis 1%, preferably 0.5%.

Further, as ingredients in a steel sheet, even if Ni, Cu, Sn, Sb, arefurther included, the effect of the present invention is not obstructed.Ni is a useful element from the viewpoint of not only the hardenabilityof steel sheet, but also the low temperature toughness which in turnleads to improvement of the impact resistance. It exhibits this effectat 0.01% or more of Ni. However, even if adding Ni in over 5%, theeffect becomes saturated and the cost rises, so N may be added in therange of 0.01 to 5%. Cu is also a useful element from the viewpoint ofnot only the hardenability of steel sheet, but also the toughness. Itexhibits this effect at 0.1% or more of Cu. However, even if adding Cuin over 3%, the effect becomes saturated and the cost rises. Not onlythat, deterioration of the slab properties and cracks and defects in thesteel sheet at the time of hot rolling are caused, so Cu should be addedin 0.01 to 3% in range. Furthermore, Sn and Sb are both elements whichare effective for improving the wettability and bondability of theplating with respect to the steel sheet. An amount of 0.005% to 0.1% canbe added. If both are amounts of less than 0.005%, no effect can berecognized, while if over 0.1% is added, defects easily are caused atthe time of production and, further, a drop in toughness is caused, sothe upper limits of the amount of Sn and the amount of Sb are 0.1%.

Further, the other ingredients are not particularly prescribed.Sometimes Zr, As, and other elements enter from the iron scrap, but ifin the usual range, they do not affect the properties of the steel whichis used for the present invention.

Next, the method of production of a hot stamped high strength part willbe explained.

The aluminum plated steel sheet for hot stamped member use which is usedin the present invention is produced by taking cold rolled steel sheetwhich has been obtained by hot rolling steel, then cold rolling it, andplating it on a hot dipping line with an annealing temperature of 670 to760° C. and a furnace time in the reducing furnace of 60 sec or less totreat the steel sheet with aluminum plating which contains Si: 7 to 15%.It is effective to make the skin pass rolling rate after aluminumplating 0.1 to 0.5%.

The annealing temperature of the hot dipping line has an effect on theshape of the steel sheet. If the annealing temperature is raised, thesteel sheet easily warps in the C direction. As a result, at the time ofaluminum plating, the difference in plating coating deposition amountsat the center part of the steel sheet in the width direction and nearthe edges will easily become larger. From this viewpoint, the annealingtemperature is preferably 760° C. or less. Further, if the annealingtemperature is too low, the temperature of the sheet when being dippedin the aluminum plating bath falls too much and dross defects easily arecaused, so the lower limit of the annealing temperature is 670° C.

The furnace time in the reducing furnace affects the aluminum platingproperties. Si, Cr, Al, and other elements which oxidize more easilythan Fe easily oxidize in the reducing furnace at the steel sheetsurface and obstruct the reaction between the aluminum plating bath andthe steel sheet. In particular, if the furnace time in the reducingfurnace is long, this effect becomes remarkable, so the furnace time ispreferably 60 sec or less. Note that the lower limit of the furnace timeis not particularly defined, but 30 sec or more is preferable.

After the aluminum plating, for shape adjustment etc., the sheet isrolled by skin pass rolling, but the rolling rate at this time affectsthe alloying of the aluminum plating layer at the time of hot stamping.Due to the rolling, strain is introduced into the steel sheet andplating layer. This is believed to be a result of this. If the rollingrate is high, the alloy layer after hot stamping tends to become smallerin crystal grain size, but it is not preferable if the rolling rate ismade too low since the alloy layer which is produced is given cracks.For this reason, the rolling rate is preferably made 0.1 to 0.5%.

Further, after the aluminum plating, box annealing can be performed tomake the aluminum plating layer alloyed. At this time, to promote thealloying, the steel preferably is made to include Cr, Mo, etc. The boxannealing is for example performed at 650° C. for 10 hours or so.

The thus obtained aluminum plated steel sheet can be rapidly heated inthe subsequent hot stamping step by a 50° C./sec or more temperatureelevation rate. Further, rapid heating is effective for making the meanlinear intercept length of the crystal grains in a phase containing Al:40 to 65% in the Al—Fe alloy layer 3 to 20 μm. The heating system is notparticularly limited. The usual furnace heating or an infrared type ofheating system using radiant heat may be used. Further, it is alsopossible to use ohmic heating or high frequency induction heating oranother heating system using electricity which enables rapid heating bya temperature elevation rate of 50° C./sec or more.

The upper limit of the temperature elevation rate is not particularlydefined, but when using the above ohmic heating or high frequencyinduction heating or other heating system, due to the performance of thesystems, 300° C./sec or so becomes the upper limit.

Further, at this heating step, the peak sheet temperature is preferablymade 850° C. or more. The peak sheet temperature is made 850° C. or moreso as to heat the steel sheet to the austenite region and promotesufficient alloying of the aluminum plating layer up to the surface.

Next, the aluminum plated steel sheet in the heated state is hot stampedto a predetermined shape between a pair of top and bottom forming dies.After being formed, it is held stationary at the press bottom deadcenter for several seconds to quench it by cooling by contact with theforming dies and thereby obtain the hot stamped high strength part ofthe present invention.

The hot stamped part was welded, chemically converted, painted byelectrodeposition, etc. to obtain the final product. Usually, cationicelectrodeposition painting is used. The film thickness becomes 1 to 30μm or so. After the electrodeposition painting, an intermediatepainting, top painting, and other painting are sometimes also applied.

EXAMPLES

Below, examples will be used to explain the present invention in furtherdetail.

Example 1

After the usual hot rolling step and cold rolling step, a cold rolledsteel sheet of the steel ingredients such as shown in Table 1 (sheetthickness 1.4 mm) was covered by hot dip aluminum plating containing Si.For the hot dip aluminum plating, a nonoxidizing furnace-reducingfurnace type of line was used. After the plating, gas wiping was used toadjust the plating coating deposition amount to a total for the twosides of 160 g/m², then the sheet was cooled. At this time, as theplating bath composition, there were (A): Al-7% Si-2% Fe, bathtemperature 660° C., and (B): Al-11% Si-2% Fe, bath temperature 640° C.The plating bath conditions correspond to the phases at the aluminumplating conditions A and B of FIG. 4. It should be noted that the Fe inthe bath is unavoidable Fe which is supplied from the plating equipmentand strips in the bath. Further, the annealing temperature was made 720°C. and the furnace time in the reducing furnace was made 45 sec. Thealuminum plated steel sheet was generally good in appearance with nononplating defects etc.

The thus prepared test piece was evaluated for post paintinganticorrosion property. The hot stamping was performed using a usualfurnace heating means. The temperature elevation rate of the aluminumplated steel sheet was approximately 5° C./sec. A 250×300 mm large testpiece was heated in the air. The piece was elevated in temperature overapproximately 3 minutes, then was held for approximately 1 minute, thenremoved from the furnace and cooled down to approximately 700° C. intemperature, formed into a hat shape, and cooled in the die. At thistime, the cooling rate was approximately 200° C./sec. As shown in Table2, the heating temperature of the test piece was changed in various waysto control the structure of the aluminum plating layer after alloying.

The vertical wall part of the hat shaped part was cut out to 50×100 mmand evaluated for post painting anticorrosion property. The chemicalconversion solution PB-SX35 made by Parkerizing used for chemicalconversion, then the cationic electrodeposition paint Powernix 110 madeby Nippon Paint was painted to give an approximately 20 μm thickness.After that, a cutter was used to cross-cut this film, then a compositecorrosion test defined by the Society of Automobile Engineers of Japan(JASO M610-92) was performed for 180 cycles (60 days). The extent ofblistering from a cross-cut (maximum blistering at the cross-cut(maximum blister width at one side) was measured. At this time, theblister width of general rust-proof steel sheet, that is, GA (hot dipgalvannealed steel sheet) (amount of deposition of 45 g/m² at one side)was 5 mm.

The post painting anticorrosion property was evaluated as “very good”with a blister width of 4 mm or less, as “good” with a blister width ofover 4 mm to 6 mm, and as “poor” with a blister width of over 6 mm.

Regarding evaluation of the spot weldability, this has to be performedby a flat sheet, so a 400×500 mm plate shaped die was used. The usualfurnace heating means was used, 400×500 mm aluminum plated steel sheetwas heated by a temperature elevation rate of approximately 5° C./sec inthe air, the sheet was evaluated in temperature over approximately 3minutes, then was held for approximately 1 minute, then was taken out ofthe furnace, cooled in the air down to approximately 700° C. intemperature, then quenched in the die. 30 mm of the two edges of thealuminum plated steel sheet, plated by Al on a hot dipping line, in thewidth direction were cut off. The rest was used for the tests. After hotstamping, the part was quenched, then a 30×50 mm weld test piece was cutout and measured for suitable weld current range by a pressure of 500kgf and electrification for 10 cycles (60Hz). At this time, the lowerlimit current was made 4√t (“t” is the sheet thickness), while the upperlimit current was made the spattering. The upper limit currentvalue-lower current value was made the suitable weld current range.

The spot weldability was evaluated as “good” when over the suitable weldcurrent range 2 kA and “poor” when the suitable weld current range 2 kAor less.

Further, after Nital etching, the test piece was examined incross-section and the average value of thickness, the standard deviationof thickness (deviation in plating thickness), and the ratio of theaverage value of thickness to the standard deviation of thickness(standard deviation/average) were found for the plating thickness.Further, the alloy layer structure was examined and the mean linearintercept length of the crystal grains of a phase which contains Al: 40to 65 mass% was measured. At this time, the test piece was cut out fromthe flange part with little deformation at the hat shaped part.

Note that, the average value of plating thickness and the standarddeviation of plating thickness were determined by sampling 20×30 mm testpieces at positions 50 mm from the two edges of the steel sheet in thewidth direction, the center, and intermediate positions between thepositions 50 mm from the two edges and the center, that is, a total offive locations. The test pieces were cut, examined in cross-section,calculated for thickness at the front and back, measured for thicknessat 10 points, and calculated for average value of thickness and standarddeviation.

The aluminum plating conditions, hot stamping conditions, mean linearintercept length, average value of thickness, and results of evaluationof the post painting anticorrosion property and weldability aredescribed in Table 2.

Further, simultaneously, the cross-sectional hardness was measured by aVicker's hardness meter (load 1 kgf), but values of a hardness of 420 ormore were obtained at all measured locations.

TABLE 1 Steel ingredients (mass %) C Si Mn Al P S N Ti B Cr 0.22 0.191.24 0.04 0.02 0.014 0.005 0.02 0.003 0.12

TABLE 2 Plating Plating Mean linear Heating Holding thickness thicknessStandard intercept Post painting Spot Plating temp. time averagestandard deviation/ length anticorrosion weld- No. conditions (° C.)(sec) (μm) deviation average (μm) property ability Remarks 1 A 850 60 282.2 0.08 4 Good Good Inv. ex. 2 A 900 60 33 2.4 0.07 7 Very Good GoodInv. ex. 3 A 950 60 37 2.1 0.06 13 Very Good Good Inv. ex. 4 A 1000 6044 2.7 0.06 22 Poor Good Comp. ex. 5 A 1050 60 53 2.4 0.05 33 Poor GoodComp. ex. 6 B 850 60 28 2.3 0.08 4 Good Good Inv. ex. 7 B 900 60 32 2.30.07 5 Very Good Good Inv. ex. 8 B 950 60 35 2.5 0.07 9 Very Good GoodInv. ex. 9 B 1000 60 42 2.6 0.06 15 Very Good Good Inv. ex. 10 B 1050 6050 2.4 0.05 23 Poor Good Comp. ex.

As shown by the results of evaluation of Table 2, test pieces of thealuminum plating conditions A and B were both hot stamped under the sameconditions, but differences were observed in the obtained alloy layerstructures (mean linear intercept lengths). Examples with large meanlinear intercept lengths fell relatively in post painting anticorrosionproperty. The reason is believed to be the plating cracks.

That is, the invention examples were all excellent in post paintinganticorrosion property and spot weldability, but in the comparativeexamples where the mean linear intercept lengths failed to satisfy therequirements of the present invention (Nos. 4, 5, 10), the post paintinganticorrosion property was inferior. Samples plated with Al by theconditions of A were used for rapid heating and quenching in a flatplate die. The heating method used a near infrared heating furnace. Thetemperature elevation rate at that time was 50° C./sec. The peak sheettemperature and the holding conditions were also changed to investigatethe structures of the plating layers at that time. The results and theresults of Table 2 are summarized in FIG. 4. It is shown that the meanlinear intercept length is dependent on the plating conditions and theheating conditions.

Example 2

Cold rolled steel sheets of the various steel ingredients (A to I) whichare shown in Table 3 (sheet thickness 1 to 2 mm) were used for aluminumplating in the same way as in Example 1. In this example, the annealingtemperature and the reducing furnace time at this time were changed. Asthe aluminum plating bath composition, by mass %, Si: 9% and Fe: 2% werecontained. The bath temperature was 660° C. and the deposition afterplating was adjusted by the gas wiping method to a total of the twosurfaces of 160 g/m².

After this, a method similar to Example 1 was used to make the heatingtemperature at the time of hot stamping 950° C. for quenching. Afterthat, the post painting anticorrosion property and the spot weldabilitywere evaluated. The method of evaluation was the same as in Example 1.The Vicker's hardness was 420 or more in all cases.

TABLE 3 Steel ingredients (mass %) C Si Mn Al P S N Ti B Cr Mo Others A0.23 0.24 1.52 0.041 0.067 0.071 0.005 0.092 0.006 — — B 0.21 0.39 0.330.041 0.009 0.053 0.003 0.033 0.0091 2.624 0.122 C 0.24 0.03 2.49 0.0380.032 0.018 0.004 0.099 0.0063 0.001 0.375 D 0.36 0.63 1.81 0.013 0.0710.053 0.005 0.089 0.0064 0.904 0.295 W: 0.01 E 0.16 0.21 0.84 0.0510.023 0.038 0.002 0.020 0.0017 2.3  0.233 Ni: 0.04 F 0.19 0.25 2.250.044 0.099 0.063 0.003 0.066 0.0026 2.156 0.255 Cu: 0.02 G 0.19 0.751.232 0.067 0.069 0.055 0.004 0.026 0.005 2.604 0.032 H 0.30 0.19 0.910.03 0.01 0.019 0.003 — — — — I 0.17 0.20 0.85 0.052 0.021 0.028 0.0020.021 0.0015 2.1  — Ni: 0.04 Sb: 0.01

TABLE 4 Reducing Plating Plating Mean linear Sheet Annealing furnacethickness thickness Standard intercept Post painting Spot thicknesstemp. time average standard deviation/ length anticorrosion weld- No.Steel (mm) (° C.) (sec) (μm) deviation average (μm) property abilityRemarks 1 A 1.2 740 40 28 2.5 0.09 12 Very Good Good Inv. ex. 2 A 1.6740 50 29 3.1 0.11 12 Very Good Good Inv. ex. 3 A 2.0 740 55 29 3.7 0.1312 Very Good Good Inv. ex. 4 A 2.0 760 55 29 4.5 0.16 12 Very Good PoorComp. ex. 5 B 1.6 730 50 28 3.0 0.11 13 Very Good Good Inv. ex. 6 C 1.6710 50 29 2.9 0.10 12 Very Good Good Inv. ex. 7 D 1.6 720 50 29 3.3 0.1112 Very Good Good Inv. ex. 8 E 1.6 730 50 28 3.2 0.11 13 Very Good GoodInv. ex. 9 F 1.6 740 50 28 3.0 0.11 12 Very Good Good Inv. ex. 10 G 2.0740 65 28 4.4 0.16 12 Poor Poor Comp. ex. 11 H 1.2 740 40 28 2.6 0.10 12Very Good Good Inv. ex. 12 I 1.6 740 50 28 3.2 0.11 12 Very Good GoodInv. ex.

In Example 2, the ingredients of the steel used, the sheet thickness,and the aluminum plating bath components were changed. As shown by theresults of evaluation of Table 4, a trend was observed where if thesheet thickness becomes larger, the standard deviation of the platingthickness becomes larger and, further, if the annealing temperaturebecomes higher, the standard deviation of the plating thickness becomeslarger. If the standard deviation is large, the suitable weld currentrange is narrow and spattering was easily generated in spot welding.Further, in a system of ingredients with high Si such as the SteelIngredients G, if the furnace time in the reducing furnace is long (65sec), nonplating defects are deemed to occur and the post paintinganticorrosion property fell.

That is, as shown by the results of evaluation of Table 4, the inventionexamples were all excellent in post painting anticorrosion property andspot weldability, but in a comparative example where the ratio of theaverage value of thickness to the standard deviation of thickness(standard deviation/average) exceeds 0.15 (No. 4), the spot weldabilitywas inferior. Further, in a comparative example where the reducingfurnace time was long and the standard deviation/average exceeded 0.15(No. 10), both the post painting anticorrosion property and spotweldability were inferior.

Example 3

The aluminum plated steel sheets of Nos. 2 and 5 of Table 4 of Example 2were box annealed to alloy the aluminum plating layers. At this time,No. 2 corresponded to the Steel Ingredients A and No. 5 to the SteelIngredients B. These differed in the amounts of Cr in the steel. At thistime, in No. 2 (Steel Ingredients A), at the time of box annealing, AlNwas formed near the interface of the aluminum plating layer and thesteel sheet and the aluminum plating layer could not be sufficientlyalloyed. In No. 5 (Steel Ingredients B), alloying was possible. UsingNo. 5, an ohmic heating means was used to raise the temperature by atemperature elevation rate of 200° C./sec up to 950° C., then the sheetwas quenched without holding. The box annealing caused the aluminumplating layer to become alloyed, so even after ohmic heating, thethickness of the Al—Fe alloy layer was constant. The post paintinganticorrosion property and spot weldability were evaluated by similarmethods to Example 1, whereupon the post painting anticorrosion propertywas evaluated as being “very good” and the spot weldability as being“good”, that is, excellent properties were shown. The Vicker's hardnesswas also shown to be 482.

Example 4

The steel of Table 1 of Example 1 was used for aluminum plating underthe aluminum plating conditions B of Example 1. At this time, theplating coating deposition amount was adjusted to a total of the twosides of 80 to 160 g/m². Furthermore, after the aluminum plating, amixture of a finely dispersed ZnO aqueous solution (Nanotech Slurry madeby C. I. Kasei) and a urethane-based water-soluble resin was coated by aroll coater and dried at 80° C. At this time, the amounts of depositionof the ZnO film were, converted to Zn, 0.5 to 3 g/m². These test pieceswere hot stamping and quenched.

As the hot stamping conditions at this time, in addition to the furnaceheating which is shown in Example 1, an infrared heating furnace wasalso used. The holding time in the case of furnace heating was 60 sec,while in the case of infrared heating was also 60 sec. Note that, thetemperature elevation rate in the infrared heating was approximately 19°C./sec. The thus prepared test piece was evaluated by the same method asin Example 1. The results of evaluation at this time are shown in Table5. The Vicker's hardness was 420 or more in all cases.

TABLE 5 Plating Zn Plating Plating Mean linear Post depositiondeposition Heating thickness thickness Standard intercept painting Spotamount amount Heating temp. average standard deviation/ lengthanticorrosion weld- No. (g/m²) (g/m²) method (° C.) (μm) deviationaverage (μm) property ability Remarks 1 80 1.0 Furnace 900 15 1.1 0.07 9Very Good Good Inv. ex. 2 80 1.0 Infrared 950 14 1.2 0.09 11 Very GoodGood Inv. ex. 3 80 2.0 Infrared 950 14 1.1 0.08 11 Very Good Good Inv.ex. 4 80 3.0 Infrared 950 15 1.3 0.09 10 Very Good Good Inv. ex. 5 1200.5 Infrared 900 23 2.0 0.09 11 Very Good Good Inv. ex. 6 160 0.5Infrared 900 29 2.4 0.08 12 Very Good Good Inv. ex. 7 160 1.0 Infrared900 29 2.3 0.08 12 Very Good Good Inv. ex.

Test pieces given a ZnO film exhibited excellent post paintinganticorrosion property even with a small deposition amount. Further, thespot weldability was also excellent.

1-7. (canceled)
 8. A method of production of an aluminum plated steelsheet for a hot stamped high strength part, comprising steps of:providing an aluminum plated steel sheet obtained characterized by hotrolling a steel which comprises chemical ingredients which comprise, bymass %, C: 0.1 to 0.5%, Si: 0.01 to 0.7%, Mn: 0.2 to 2.5%, Al: 0.01 to0.5%, P: 0.001 to 0.1%, S: 0.001 to 0.1%, N: 0.0010% to 0.05%, and abalance of Fe and unavoidable impurities, cold rolling said hot rolledsteel to obtain a cold rolled steel sheet, heating said cold rolledsteel sheet on a hot dipping line to an annealing temperature of 670 to760° C., holding said heated steel sheet in a reducing furnace for 60sec or less, and aluminum plating said steel sheet; and temper rollingsaid aluminum plated steel sheet to give a rolling rate of 0.5 to 2%;raising the temperature of said temper rolled aluminum plated steelsheet by a temperature elevation rate of 3 to 200° C./sec; hot stampingthe aluminum plated steel sheet under conditions of a Larson-Millerparameter (LMP) expressed by the following formula:LMP=T(20+log t) (wherein, T: heating temperature of aluminum platedsteel sheet (absolute temperature K), t: holding time in heating furnaceafter reaching target temperature (hrs)) of 20000 to 23000; andquenching said aluminum plated steel sheet after hot stamping at a 20 to500° C./sec cooling rate in the die.
 9. The method of production of analuminum plated steel sheet for a hot stamped high strength part as setforth in claim 8 characterized in that said steel furthermore comprises,by mass %, one or more of the elements selected from Cr: over 0.4 to 3%,Mo: 0.005 to 0.5%, B: 0.0001 to 0.01%, W: 0.01 to 3%, V: 0.01 to 2%, Ti:0.005 to 0.5%, Nb: 0.01 to 1% Ni: 0.01 to 5%, Cu: 0.1 to 3%, Sn: 0.005%to 0.1%, and Sb: 0.005% to 0.1%.
 10. The method of production of analuminum plated steel sheet for a hot stamped high strength part as setforth in claim 8 or 9 characterized in that in the temperature elevationrate in said hot stamping step is 4 to 200° C./sec.
 11. The method ofproduction of an aluminum plated steel sheet for a hot stamped highstrength part as set forth in any one of claims 8 to 10 characterized inthat in the step of producing said aluminum plated steel sheet, aplating bath for aluminum plating comprises Si in an amount of 7 to 15%,and either a bath temperature or a sheet temperature upon entering thebath is 650° C. or less.